Compound charge transport layer for organic photovoltaic devices

ABSTRACT

Organic photovoltaic devices with compound charge transport layers are described herein. One such device includes a substrate, a first electrode coupled to the substrate, a second electrode disposed above the first electrode, and photoactive layers disposed between the first electrode and the second electrode. The device further includes a compound charge transport layer disposed between the photoactive layers and either the first electrode or the second electrode. The compound charge transport layer includes a charge transport layer and a metal-oxide interlayer disposed between the charge transport layer and the photoactive layers. The charge transport layer may be a hole transport layer or an electron transport layer.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalPatent Application No. 62/970,010, filed Feb. 4, 2020, the entirecontent of which is incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

Photovoltaic devices are commonly employed to convert light intoelectricity by using the photovoltaic effect, in which absorbed lightcauses the excitation of an electron or other charge carrier to ahigher-energy state. The separation of charge carriers of opposite typesleads to a voltage that can be utilized by an external circuit.Photovoltaic devices, such as photovoltaic solar cells, can be packagedtogether to constitute a photovoltaic array of a larger photovoltaicsystem, such as a solar panel. The use of photovoltaic systems togenerate electricity is an important form of renewable energy thatcontinues to become a mainstream electricity source worldwide.

The surface area necessary to take advantage of solar energy remains anobstacle to offsetting a significant portion of non-renewable energyconsumption. For this reason, low-cost, transparent, organicphotovoltaic (OPV) devices that can be integrated onto window panes inhomes, skyscrapers, and automobiles are desirable. For example, windowglass utilized in automobiles and architecture are typically 70-80% and40-80% transmissive, respectively, to the visible spectrum, e.g., lightwith wavelengths from about 450 to 650 nm. The limited mechanicalflexibility, high module cost and, more importantly, the band-likeabsorption of inorganic semiconductors limit their potential utility totransparent solar cells.

SUMMARY OF THE INVENTION

The present disclosure relates to organic photovoltaic devices (OPVs),and in some embodiments, visibly transparent photovoltaic devicesincorporating visibly transparent photoactive compounds. The visiblytransparent photoactive compounds absorb light more strongly in thenear-infrared and/or ultraviolet regions and less strongly in thevisible region, permitting their use in visibly transparent photovoltaicdevices. The disclosed visibly transparent photovoltaic devices includevisibly transparent electrodes with visibly transparent photoactivematerials positioned between the visibly transparent electrodes.

The optical characteristics of organic and molecular semiconductorsresult in absorption spectra that are highly structured with absorptionminima and maxima that are uniquely distinct from the band absorption oftheir inorganic counterparts. However, while a variety of organic andmolecular semiconductors exist, many exhibit strong absorption in thevisible spectrum and thus are not optimal for use in window glass-basedphotovoltaics.

A summary of the present invention is provided in reference to variousexamples given below. As used below, any reference to a series ofexamples is to be understood as a reference to each of those examplesdisjunctively (e.g., “Examples 1-4” is to be understood as “Examples 1,2, 3, or 4”).

Example 1 is an organic photovoltaic device comprising: a substrate; afirst electrode coupled to the substrate; a second electrode disposedabove the first electrode; one or more photoactive layers disposedbetween the first electrode and the second electrode; and a compoundcharge transport layer disposed between the one or more photoactivelayers and either the first electrode or the second electrode, whereinthe compound charge transport layer includes: a charge transport layer;and a metal-oxide interlayer (IL) disposed between the charge transportlayer and the one or more photoactive layers.

Example 2 is the organic photovoltaic device of example(s) 1, furthercomprising: a second compound charge transport layer coupled to thecompound charge transport layer, wherein the second compound chargetransport layer includes: a second charge transport layer; and a secondmetal-oxide interlayer coupled to the second charge transport layer anddisposed between the second charge transport layer and the one or morephotoactive layers.

Example 3 is the organic photovoltaic device of example(s) 1, whereinthe first electrode is an anode and the second electrode is a cathode.

Example 4 is the organic photovoltaic device of example(s) 1, whereinthe charge transport layer is a hole transport layer (HTL).

Example 5 is the organic photovoltaic device of example(s) 1, furthercomprising: a metal-oxide charge-injection layer disposed between thefirst electrode and the compound charge transport layer.

Example 6 is the organic photovoltaic device of example(s) 1, whereinthe first electrode is a cathode and the second electrode is an anode.

Example 7 is the organic photovoltaic device of example(s) 1, whereinthe charge transport layer is an electron transport layer (ETL).

Example 8 is the organic photovoltaic device of example(s) 1, furthercomprising: a metal-oxide charge-injection layer disposed between thecompound charge transport layer and the second electrode.

Example 9 is the organic photovoltaic device of example(s) 1-8, whereinthe one or more photoactive layers includes a bulk heterojunctionphotoactive layer comprising a blend of an electron donor material andan electron acceptor material.

Example 10 is the organic photovoltaic device of example(s) 1-9, whereinthe organic photovoltaic device is visibly transparent.

Example 11 is the organic photovoltaic device of example(s) 1-10,wherein the charge transport layer has distinct spectral absorptionproperties from the one or more photoactive layers.

Example 12 is an organic photovoltaic device comprising: a substrate; afirst electrode coupled to the substrate; a second electrode disposedabove the first electrode; one or more photoactive layers disposedbetween the first electrode and the second electrode; and a compoundcharge transport layer disposed between the first electrode and the oneor more photoactive layers, wherein the compound charge transport layerincludes: a charge transport layer; and a metal-oxide interlayer coupledto the charge transport layer and disposed between the charge transportlayer and the one or more photoactive layers.

Example 13 is the organic photovoltaic device of example(s) 12, furthercomprising: a second compound charge transport layer coupled to thecompound charge transport layer, wherein the second compound chargetransport layer includes: a second charge transport layer; and a secondmetal-oxide interlayer coupled to the second charge transport layer anddisposed between the second charge transport layer and the one or morephotoactive layers.

Example 14 is the organic photovoltaic device of example(s) 12, whereinthe first electrode is an anode, the second electrode is a cathode, andthe charge transport layer is a hole transport layer.

Example 15 is the organic photovoltaic device of example(s) 14, furthercomprising: a metal-oxide hole-injection layer (HIL) disposed betweenthe first electrode and the hole transport layer.

Example 16 is the organic photovoltaic device of example(s) 12, whereinthe first electrode is a cathode, the second electrode is an anode, andthe charge transport layer is an electron transport layer.

Example 17 is the organic photovoltaic device of example(s) 16, furthercomprising: a metal-oxide electron-injection layer (EIL) disposedbetween the first electrode and the electron transport layer.

Example 18 is the organic photovoltaic device of example(s) 12-17,wherein the one or more photoactive layers includes a bulkheterojunction photoactive layer comprising a blend of an electron donormaterial and an electron acceptor material.

Example 19 is the organic photovoltaic device of example(s) 12-18,further comprising: a p-phenylene layer disposed between the metal-oxideinterlayer and the one or more photoactive layers.

Example 20 is the organic photovoltaic device of example(s) 12-19,wherein the organic photovoltaic device is visibly transparent.

Example 21 is the organic photovoltaic device of example(s) 12-20,wherein the charge transport layer has distinct spectral absorptionproperties from the one or more photoactive layers.

Example 22 is an organic photovoltaic device comprising: a substrate; afirst electrode coupled to the substrate; a second electrode disposedabove the first electrode; one or more photoactive layers disposedbetween the first electrode and the second electrode; and a compoundcharge transport layer disposed between the second electrode and the oneor more photoactive layers, wherein the compound charge transport layerincludes: a charge transport layer; and a metal-oxide interlayer coupledto the charge transport layer and disposed between the charge transportlayer and the one or more photoactive layers.

Example 23 is the organic photovoltaic device of example(s) 22, furthercomprising: a second compound charge transport layer coupled to thecompound charge transport layer, wherein the second compound chargetransport layer includes: a second charge transport layer; and a secondmetal-oxide interlayer coupled to the second charge transport layer anddisposed between the second charge transport layer and the one or morephotoactive layers.

Example 24 is the organic photovoltaic device of example(s) 22, whereinthe first electrode is an anode, the second electrode is a cathode, andthe charge transport layer is an electron transport layer.

Example 25 is the organic photovoltaic device of example(s) 24, furthercomprising: a metal-oxide electron-injection layer disposed between thesecond electrode and the electron transport layer.

Example 26 is the organic photovoltaic device of example(s) 22, whereinthe first electrode is a cathode, the second electrode is an anode, andthe charge transport layer is a hole transport layer.

Example 27 is the organic photovoltaic device of example(s) 26, furthercomprising: a metal-oxide hole-injection layer disposed between thesecond electrode and the hole transport layer.

Example 28 is the organic photovoltaic device of example(s) 22-27,wherein the one or more photoactive layers includes a bulkheterojunction photoactive layer comprising a blend of an electron donormaterial and an electron acceptor material.

Example 29 is the organic photovoltaic device of example(s) 22-28,further comprising: a p-phenylene layer disposed between the metal-oxideinterlayer and the one or more photoactive layers.

Example 30 is the organic photovoltaic device of example(s) 22, whereinthe organic photovoltaic device is visibly transparent.

Example 31 is the organic photovoltaic device of example(s) 22, whereinthe charge transport layer has distinct spectral absorption propertiesfrom the one or more photoactive layers.

Example 32 is an organic photovoltaic device comprising: a substrate; afirst electrode coupled to the substrate; a second electrode disposedabove the first electrode; one or more photoactive layers disposedbetween the first electrode and the second electrode; a first compoundcharge transport layer disposed between the first electrode and the oneor more photoactive layers; and a second compound charge transport layerdisposed between the second electrode and the one or more photoactivelayers, wherein each compound charge transport layer includes: a chargetransport layer; and a metal-oxide interlayer coupled to the chargetransport layer and disposed between the charge transport layer and theone or more photoactive layers.

Example 33 is the organic photovoltaic device of example(s) 32, whereinthe first electrode is an anode, the second electrode is a cathode, thefirst charge transport layer is a hole transport layer, and the secondcharge transport layer is an electron transport layer.

Example 34 is the organic photovoltaic device of example(s) 33, furthercomprising: a metal-oxide hole-injection layer disposed between thefirst electrode and the hole transport layer.

Example 35 is the organic photovoltaic device of example(s) 33, furthercomprising: a metal-oxide electron-injection layer disposed between thesecond electrode and the electron transport layer.

Example 36 is the organic photovoltaic device of example(s) 34, furthercomprising: a metal-oxide electron-injection layer disposed between thesecond electrode and the electron transport layer.

Example 37 is the organic photovoltaic device of example(s) 32, whereinthe first electrode is an cathode, the second electrode is an anode, thefirst charge transport layer is an electron transport layer, and thesecond charge transport layer is a hole transport layer.

Example 38 is the organic photovoltaic device of example(s) 37, furthercomprising: a metal-oxide electron-injection layer disposed between thefirst electrode and the electron transport layer.

Example 39 is the organic photovoltaic device of example(s) 37, furthercomprising: a metal-oxide hole-injection layer disposed between thesecond electrode and the hole transport layer.

Example 40 is the organic photovoltaic device of example(s) 38, furthercomprising: a metal-oxide hole-injection layer disposed between thesecond electrode and the hole transport layer.

Example 41 is the organic photovoltaic device of example(s) 32-40,wherein the one or more photoactive layers includes a bulkheterojunction photoactive layer comprising a blend of an electron donormaterial and an electron acceptor material.

Example 42 is the organic photovoltaic device of example(s) 32-41,further comprising: a p-phenylene layer disposed between the metal-oxideinterlayer of the first compound charge transport layer and the one ormore photoactive layers.

Example 43 is the organic photovoltaic device of example(s) 32-41,further comprising: a p-phenylene layer disposed between the metal-oxideinterlayer of the second compound charge transport layer and the one ormore photoactive layers.

Example 44 is the organic photovoltaic device of example(s) 32-41,further comprising: a first p-phenylene layer disposed between themetal-oxide interlayer of the second compound charge transport layer andthe one or more photoactive layers.

Example 45 is the organic photovoltaic device of example(s) 32-44,wherein the organic photovoltaic device is visibly transparent.

Example 46 is the organic photovoltaic device of example(s) 32-45,wherein the charge transport layers have distinct spectral absorptionproperties from the one or more photoactive layers.

Example 47 is a method of making an organic photovoltaic device, themethod comprising: providing a substrate; forming a first electrode overthe substrate; forming a second electrode over the first electrode;forming one or more photoactive layers between the first electrode andthe second electrode; and forming a compound charge transport layerbetween the one or more photoactive layers and either the firstelectrode or the second electrode, wherein the compound charge transportlayer includes: a charge transport layer; and a metal-oxide interlayercoupled to the charge transport layer and disposed between the chargetransport layer and the one or more photoactive layers.

Example 48 is the method of example(s) 47, further comprising: forming asecond compound charge transport layer coupled to the compound chargetransport layer, wherein the second compound charge transport layerincludes: a second charge transport layer; and a second metal-oxideinterlayer coupled to the second charge transport layer and disposedbetween the second charge transport layer and the one or morephotoactive layers.

Example 49 is the method of example(s) 47, wherein the first electrodeis an anode and the second electrode is a cathode.

Example 50 is the method of example(s) 47, wherein the charge transportlayer is a hole transport layer.

Example 51 is the method of example(s) 47, further comprising: forming ametal-oxide charge-injection layer between the first electrode and thecompound charge transport layer.

Example 52 is the method of example(s) 47, wherein the charge transportlayer is an electron transport layer.

Example 53 is the method of example(s) 47, further comprising: forming ametal-oxide charge-injection layer between the compound charge transportlayer and the second electrode.

Example 54 is the method of example(s) 47-53, wherein the one or morephotoactive layers includes a bulk heterojunction photoactive layercomprising a blend of an electron donor material and an electronacceptor material.

Example 55 is the method of example(s) 47-54, wherein the organicphotovoltaic device is visibly transparent.

Example 56 is the method of example(s) 47-55, wherein the chargetransport layer has distinct spectral absorption properties from the oneor more photoactive layers.

Numerous benefits are achieved by way of the present invention overconventional techniques. For example, embodiments of the presentdisclosure enable the barrier-free coupling of dissimilar organicmaterials in the case of multilayers, which would normally be limited intheir pairing by HOMO or LUMO level offsets. Additionally, thesandwiching of the organic layers between metal oxides decouples theirenergy levels from the built-in potential of the photovoltaic cell, suchthat their HOMO or LUMO levels do not limit the open-circuit voltage.According to embodiments of the present invention, the compound chargetransport layer can provide a low-resistance and charge-selective(preferential to either holes or electrons) contact to the photovoltaicactive layers.

Different benefits can be achieved by varying the implementation of the“compound” charge transport layer. The compound charge transport layersmay be included as a single structure or may be repeated as multilayersin a stack. The metal oxide material may be varied between layers orkept consistent, and the organic layers may be kept consistent or variedin the case of multilayers. The individual organic layers and metaloxides may themselves be multilayers (e.g. two or more layers ofdifferent organics or metal oxides, respectively). Optionally, theorganic layer(s) may be extrinsically doped to further reduce electricalresistance.

These and other embodiments and aspects of the invention along with manyof its advantages and features are described in more detail inconjunction with the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a simplified schematic diagram of a visiblytransparent photovoltaic device according to an embodiment of thepresent invention.

FIGS. 2A-2E illustrate various example junction configurations for aphotoactive layer.

FIG. 3 illustrates a simplified plot of the solar spectrum, human eyesensitivity, and exemplary visibly transparent photovoltaic deviceabsorption as a function of wavelength.

FIG. 4 illustrates a schematic energy level diagram overview foroperation of an example organic photovoltaic device without any chargetransport layers, such as a visibly transparent photovoltaic device.

FIG. 5A-5D illustrate various plots showing example absorption bands fordifferent electron donor and electron acceptor configurations usefulwith visibly transparent photovoltaic devices.

FIG. 6 illustrates a schematic energy level diagram of an exampleorganic photovoltaic device exhibiting a charge extraction barrier forholes due to a misalignment of the HOMO levels at the HTL/donorinterface.

FIG. 7 illustrates a schematic energy level diagram of an exampleorganic photovoltaic device exhibiting a charge injection barrier forholes due to a misalignment of the HOMO levels at the HTL/donorinterface.

FIG. 8 illustrates a schematic energy level diagram of an exampleorganic photovoltaic device employing a compound HTL that exhibitsbarrier-free charge transport, according to some embodiments of theinvention.

FIG. 9 illustrates a device structure for an example organicphotovoltaic device having a single compound HTL.

FIG. 10 illustrates a device structure for an example organicphotovoltaic device having a multilayer compound HTL.

FIG. 11 illustrates a device structure for an example organicphotovoltaic device having a single compound ETL.

FIG. 12 illustrates a device structure for an example organicphotovoltaic device having a multilayer compound ETL.

FIG. 13 illustrates a device structure for an example organicphotovoltaic device having a single compound HTL with a varyingmetal-oxide interlayer thickness.

FIG. 14A-14D illustrate some example materials that can be utilized inan HTL.

FIG. 15 illustrates energy levels associated with the various materialsof FIG. 14.

FIG. 16A-16D illustrate plots showing measured power conversionefficiency as a function of metal-oxide interlayer thickness for variousorganic photovoltaic devices employing the device structure of FIG. 13and the HTL materials of FIGS. 14 and 15.

FIG. 17 illustrates a device structure for an example organicphotovoltaic device having a single compound HTL with a varying organicHTL thickness.

FIG. 18A-18C illustrate plots showing experimental performance and colorvalues acquired for the transparent organic photovoltaic devices of FIG.17 as a function of organic HTL thickness.

FIG. 19A-19C illustrate device structures for example organicphotovoltaic devices having multilayer compound HTLs with a fixed totalHTL thickness split into a varying number of multilayers.

FIG. 20 illustrates a plot showing measured power conversion efficiencyas a function of metal-oxide interlayer thickness for a varying numberof multilayers and employing the device structures of FIG. 19A-19C.

FIG. 21 illustrates a device structure for an example organicphotovoltaic device having a multilayer compound HTL comprised of twodifferent HTL materials.

FIG. 22A-22C illustrate plots showing experimental performance and colorvalues acquired for transparent organic photovoltaic devices with amultilayer compound HTL comprised of two different HTL materials andemploying the device structure of FIG. 21.

FIG. 23 illustrates a device structure for an example organicphotovoltaic device having a single compound HTL and a stacked BHJactive layer architecture.

FIG. 24A-24C illustrate plots showing experimental performance and colorvalues acquired for an organic photovoltaic device with a stacked BHJactive layer as a function of organic HTL thickness and employing thedevice structure of FIG. 23.

FIG. 25 illustrates a device structure for an example organicphotovoltaic device having a single compound HTL in an invertedarchitecture.

FIG. 26 illustrates a device structure for an example organicphotovoltaic device having a single compound ETL in an invertedarchitecture.

FIG. 27 illustrates a device structure for an example organicphotovoltaic device having a single compound HTL in an invertedarchitecture.

FIGS. 28A and 28B illustrate plots showing experimental values acquiredfor an organic photovoltaic device having a single compound HTL in aninverted architecture and employing the device structure of FIG. 27.

FIG. 29 illustrates a device structure for an example organicphotovoltaic device having both a compound HTL and a compound ETL in aconventional architecture.

FIG. 30 illustrates a device structure for an example organicphotovoltaic device having both a compound HTL and a compound ETL in aninverted architecture.

FIG. 31 illustrates a method for making a photovoltaic device.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Charge transport layers in organic photovoltaics (OPVs) can provide avariety of benefits to device performance. Sufficiently wide-bandgaplayers can block excitons in the active layers from reaching theelectrodes (where parasitic exciton recombination occurs) andadditionally serve to inhibit metal penetration into the active layerswhen applied below an evaporated top metal contact. Charge transportlayers may be included to improve charge selectivity (preferential holeor electron transport) to the contacts. In opaque cells with a thickmetal electrode, or in the case of devices exhibiting strong opticalinterference effects, transport layers may be used to tune the opticalcavity to tailor absorption in the active layers at specificwavelengths. This approach has been used to produce spectrally narroworganic photodetectors (OPDs) and is commonly used in OPVs to maximizeabsorption and photocurrent more broadly.

For transparent OPVs, charge transport layers may be used withstructured absorption in the ultraviolet (UV), visible, or near-infrared(NIR) wavelengths to tailor the transmitted and reflected spectra fromthe device. While usually not contributing to photocurrent, these layerscan be used as complimentary absorbers to the active layer materialssuch that a neutral (or desired) color is produced. When used as NIRabsorbers, these layers may reduce the transmitted solar irradiancethrough the transparent OPV, thus reducing the overall solar heat gainand improving thermal efficiency for applications such as architecturalglass.

Conventional approaches to charge transport layers typically requiresufficient alignment of energy levels between neighboring organic layersto avoid injection/extraction barriers and the buildup of chargecarriers. In the case of organic hole transport layers (HTLs), thehighest occupied molecular orbital (HOMO) is typically aligned to theHOMO of the donor(s) in the neighboring device active layers.Conversely, in the case of electron transport layers (ETLs), the lowestunoccupied molecular orbital (LUMO) level is sufficiently aligned to theLUMO of the acceptor(s) in the neighboring device active layers. In someinstances, charge transport layers with the opposite polarity may beused—for example, a material such as Ru(acac)₃ with a shallow HOMO levelaligned to the LUMO of the acceptor(s) in the active layer may be usedas a hole-conducting (instead of electron-conducting) transport layer tothe cathode. Similarly, materials such as HAT-CN with a deep LUMO levelaligned to the HOMO of the donor(s) in the active layer may be used asan electron-conducting (as opposed to hole-conducting) transport layerto the anode.

One modification to the above approaches for HTLs and ETLs is to usedopants to increase the organic transport layer's conductivity and tunethe work function such that any energy level misalignment effects arereduced or minimized. This approach may be characterized by severallimitations. First, energy level alignment may still be an issue in thecase of significantly mismatched HOMO or LUMO levels between the dopedtransport layer and the active layer. Second, the doping efficiency ofmost dopants is usually quite low. For example, in the case of smallmolecule organic dopants less than 10% of dopant molecules maycontribute charge. Third, most dopants are not universal to all organichost molecules, so modifications to the transport layers may require newdopants to be effective. Fourth, doping organic layers typicallyintroduces new absorption features due to cationic or anionic radicalsin both the charged dopant species and the high density of chargesintroduced to the host molecules. This can impact color tuning in thecase of transparent OPVs or introduce absorption bands overlapping theactive layers' absorption, parasitically dissipating some of theincident light in the doped layers. Finally doped transport layers aretypically less stable than undoped layers due to the fact that chargedspecies are highly reactive to both oxygen and moisture and can undergochemical reactions.

In many cases, charge transport layers are connected to the electrode bya charge injection layer. In the case of an HTL or ETL this chargeinjection layer would commonly be referred to as a hole-injection layer(HIL) or electron-injection layer (EIL), respectively. Often HILs andEILs are composed of conductive metal oxides such as MoO₃, WoO₃, or NiO,or polymers such as PEDOT:PSS or PETE. In all cases, these layersprovide an ohmic contact to the transport layers for efficient chargeinjection and extraction. For the purposes of this disclosure, the HILor EIL are considered to be part of the anode or cathode, respectively.

In the present disclosure, the concept of a “compound” charge transportlayer is introduced which may comprise an HTL or ETL sandwiched betweenthe electrode (anode or cathode, respectively) and a conductive metaloxide. Often on the electrode side, a conductive metal oxide acts as atraditional charge injection layer (e.g. HIL). However on the activelayer side of the compound HTL (or ETL), the conductive metal oxideinterlayer provides ohmic contact between the organic charge transportlayer and the active layer. This essentially decouples the energy levelsof the transport layer and the active layer, fully removing therequirement of energy level alignment. The sandwiching of the organictransport layers has the additional benefit of providing someinterfacial doping at the organic/oxide interfaces, which helps toimprove conductivity without the use and associated complications ofextrinsic doping. In the embodiments presented, HTLs comprised of smallorganic molecules are demonstrated with HOMO levels spanning ˜2 eV thatall exhibit comparable high performance.

Multiple combinations of organic transport layers (such as multipleorganic HTLs) are also demonstrated in the same device usingmultilayers, whereby distinct HTLs are connected through thin metaloxide layers and the entire compound HTL stack is sandwiched betweenmetal oxide layers. In this way, multiple absorbers or transport layerswith varying refractive index may be incorporated to tailor the color orspectral response of an OPV (or OPD) without impacting deviceperformance.

While the compound charge transport layer does not require extrinsicdoping of the organics to perform well, these approaches are notmutually exclusive. Doped organic layers may also be used as thetransport layers, which may further improve conductivity and provideadditional degrees of freedom for device engineering. Multiple HTLs aredemonstrated to function within the compound HTL structure as bothintrinsic (undoped) layers as well as extrinsically doped layers whenblended with a dopant such as a metal oxide. This highlights theversatility of the compound HTL approach—materials that would typicallybe incompatible with the paired active layers (giving low or near-zeroefficiency) are shown to work within a compound HTL structure regardlessof doping. For this reason, the transport layer within a compound HTL orETL may comprise single or blended organics, organic/inorganic blends,or combinations thereof.

Although many examples of the present disclosure focus on HTLs, thisconcept may also be extended to ETLs, provided an oxide is used withsufficient ohmic contact to the neighboring organic layers.

For transparent OPVs, charge transport layers may be used withstructured absorption in the ultraviolet (UV), visible, or near-infrared(NIR) wavelengths to tailor the transmitted and reflected spectra of thedevice. While usually not contributing to photocurrent, these layers canbe used as complimentary absorbers to the active layer materials suchthat a neutral (or desired) color is produced. When used as NIRabsorbers, these layers may reduce the transmitted solar irradiancethrough the transparent OPV, thus reducing the overall solar heat gainand improving thermal efficiency for applications such as architecturalglass. In the embodiments presented, both visibly transparent andvisibly absorbing HTL materials are utilized in compound HTLs. In thecase of UV-absorbing, visibly transparent transport layers, thereflected color is demonstrated to be adjusted with HTL thicknessindependent of the transmitted color and performance. Similarly, visiblyabsorbing HTLs with complementary absorption to the active layer aredemonstrated to neutralize transmitted color while simultaneouslyadjusting the reflected color. Combinations of both approaches are alsopresented, whereby a visibly transparent HTL is paired with a visiblyabsorbing HTL within a multilayer compound HTL. This combines theadvantages of both approaches—enabling the tuning of transmitted andreflected color independently of one another. Some of the examples ofvisibly absorbing HTLs demonstrated here exhibit NIR absorption, whichmay additionally reduce the SHGC of the transparent OPVs.

Accordingly, in various embodiments, the charge transport layer may havedistinct spectral absorption from the photoactive layers. As oneexample, the photoactive layers may have tail absorption in blue and redwhile the charge transport layer has peak absorption in the visible,complementary to active layers (e.g. in the green), the resulting effectbeing a neutralized transmitted color. As another example, thephotoactive layer may have a narrow peak absorption in a portion of thenear infrared while the charge transport layer has broad absorption inthe NIR, complementary to active layer, the resulting effect being adecreased SHGC while retaining high AVT. As another example, thephotoactive layers may have any type of absorption while the chargetransport layer has minimal absorption across the spectrum, theresulting effect being a reflected color that is modulated.

FIG. 1 illustrates a simplified schematic diagram of a visiblytransparent photovoltaic device 100, according to an embodiment of thepresent invention. As illustrated in FIG. 1, visibly transparentphotovoltaic device 100 includes a number of layers and elementsdiscussed more fully below. As discussed herein, visibly transparentindicates that the photovoltaic device absorbs optical energy atwavelengths outside the visible wavelength band of 450 nm to 650 nm, forexample, while substantially transmitting visible light inside thevisible wavelength band. As illustrated in FIG. 1, UV and/or NIR lightis absorbed in the layers and elements of the photovoltaic device whilevisible light is transmitted through the device. Thus, the discussion oftransparency provided herein should be understood as visibletransparency.

Substrate 105, which can be glass or other visibly transparent materialsproviding sufficient mechanical support to the other layers andstructures illustrated, supports optical layers 110 and 112. Theseoptical layers can provide a variety of optical properties, includingantireflection (AR) properties, wavelength selective reflection ordistributed Bragg reflection properties, index matching properties,encapsulation, or the like. Optical layers may advantageously be visiblytransparent. An additional optical layer 114 can be utilized, forexample, as an AR coating, an index matching layer, a passive infraredor ultraviolet absorption layer, etc. Optionally, optical layers may betransparent to ultraviolet and/or near-infrared light or transparent toat least a subset of wavelengths in the ultraviolet and/or near-infraredbands. Depending on the configuration, additional optical layer 114 mayalso be a passive visible absorption layer. Example substrate materialsinclude various glasses and rigid or flexible polymers. Multilayersubstrates such as laminates and the like may also be utilized.Substrates may have any suitable thickness to provide the mechanicalsupport needed for the other layers and structures, such as, forexample, thicknesses from 1 mm to 20 mm. In some cases, the substratemay be or comprise an adhesive film to allow application of the visiblytransparent photovoltaic device 100 to another structure, such as awindow pane, display device, etc.

It will be appreciated that, although the devices overall may exhibitvisible transparency, such as a transparency in the 450-650 nm rangegreater than 30%, greater than 40%, greater than 50%, greater than 60%,greater than 70%, or up to or approaching 100%, certain materials takenindividually may exhibit absorption in portions of the visible spectrum.Optionally, each individual material or layer in a visibly transparentphotovoltaic device has a high transparency in the visible range, suchas greater than 30% (i.e., between 30% and 100%). It will be appreciatedthat transmission or absorption may be expressed as a percentage and maybe dependent on the material's absorbance properties, a thickness orpath length through an absorbing material, and a concentration of theabsorbing material, such that a material with an absorbance in thevisible spectral region may still exhibit a low absorption or hightransmission if the path length through the absorbing material is shortand/or the absorbing material is present in low concentration.

As described herein and below, photoactive materials in variousphotoactive layers advantageously exhibit minimal absorption in thevisible region (e.g., less than 20%, less than 30%, less than 40%, lessthan 50%, less than 60%, or less than 70%), and instead exhibit highabsorption in the near-infrared and/or ultraviolet regions (e.g., anabsorption peak of greater than 50%, greater than 60%, greater than 70%,or greater than 80%). For some applications, absorption in the visibleregion may be as large as 70%. Various configurations of othermaterials, such as the substrate, optical layers, and buffer layers, mayallow these materials to provide overall visible transparency, eventhough the materials may exhibit some amount of visible absorption. Forexample, a thin film of a metal may be included in a transparentelectrode, such as a metal that exhibits visible absorption, like Ag orCu; when provided in a thin film configuration, however, the overalltransparency of the film may be high. Similarly, materials included inan optical or buffer layer may exhibit absorption in the visible rangebut may be provided at a concentration or thickness where the overallamount of visible light absorption is low, providing visibletransparency.

The visibly transparent photovoltaic device 100 also includes a set oftransparent electrodes 120 and 122 with a photoactive layer 140positioned between electrodes 120 and 122. These electrodes, which canbe fabricated using ITO, thin metal films, or other suitable visiblytransparent materials, provide electrical connection to one or more ofthe various layers illustrated. For example, thin films of copper,silver, or other metals may be suitable for use as a visibly transparentelectrode, even though these metals may absorb light in the visibleband. When provided as a thin film, however, such as a film having athickness of 1 nm to 200 nm (e.g., about 5 nm, about 10 nm, about 15 nm,about 20 nm, about 25 nm, about 30 nm, about 35 nm, about 40 nm, about45 nm, about 50 nm, about 55 nm, about 60 nm, about 65 nm, about 70 nm,about 75 nm, about 80 nm, about 85 nm, about 90 nm, about 95 nm, about100 nm, about 105 nm, about 110 nm, about 115 nm, about 120 nm, about125 nm, about 130 nm, about 135 nm, about 140 nm, about 145 nm, about150 nm, about 155 nm, about 160 nm, about 165 nm, about 170 nm, about175 nm, about 180 nm, about 185 nm, about 190 nm, or about 195 nm), anoverall transmittance of the thin film in the visible band may remainhigh, such as greater than 30%, greater than 40%, greater than 50%,greater than 60%, greater than 70%, greater than 80%, or greater than90%. Advantageously, thin metal films, when used as transparentelectrodes, may exhibit lower absorption in the ultraviolet band thanother semiconducting materials that may be useful as a transparentelectrode, such as ITO, as some semiconducting transparent conductingoxides exhibit a band gap that occurs in the ultraviolet band and thusare highly absorbing or opaque to ultraviolet light. In some cases,however, an ultraviolet absorbing transparent electrode may be used,such as to screen at least a portion of the ultraviolet light fromunderlying components, as ultraviolet light may degrade certainmaterials.

A variety of deposition techniques may be used to generate a transparentelectrode, including vacuum deposition techniques, such as atomic layerdeposition, chemical vapor deposition, physical vapor deposition,thermal evaporation, sputter deposition, epitaxy, etc. Solution baseddeposition techniques, such as spin-coating, slot-die coating, bladecoating, spray coating etc. may also be used in some cases. In addition,transparent electrodes may be patterned using techniques known in theart of microfabrication, including lithography, lift off, etching, etc.

Buffer layers 130 and 132 and photoactive layer 140 are utilized toimplement the electrical and optical properties of the photovoltaicdevice. These layers can be layers of a single material or can includemultiple sub-layers as appropriate to the particular application. Thus,the term “layer” is not intended to denote a single layer of a singlematerial but can include multiple sub-layers of the same or differentmaterials. In some embodiments, buffer layer 130, photoactive layer(s)140 and buffer layer 132 are repeated in a stacked configuration toprovide tandem device configurations, such as including multipleheterojunctions. In some embodiments, the photoactive layer(s) includeelectron donor materials and electron acceptor materials, also referredto as donors and acceptors. These donors and acceptors are visiblytransparent but absorb outside the visible wavelength band to providethe photoactive properties of the device.

Useful buffer layers include those that function as electron transportlayers, electron blocking layers, hole transport layers, hole blockinglayers, exciton blocking layers, optical spacers, physical bufferlayers, charge recombination layers, or charge generation layers. Bufferlayers may exhibit any suitable thickness to provide the bufferingeffect desired and may optionally be present or absent. Useful bufferlayers, when present, may have thicknesses from 1 nm to 1 μm. Variousmaterials may be used as buffer layers, including fullerene materials,carbon nanotube materials, graphene materials, metal oxides, such asmolybdenum oxide, titanium oxide, zinc oxide, etc., polymers, such aspoly(3,4-ethylenedioxythiophene), polystyrene sulfonic acid,polyaniline, etc., copolymers, polymer mixtures, and small molecules,such as bathocuproine. Buffer layers may be applied using a depositionprocess (e.g., thermal evaporation or sputtering) or a solutionprocessing method (e.g., spin coating).

Examples of materials that can be utilized as active/buffer (transportlayers)/optical materials in various embodiments of the presentinvention include near-IR absorbing materials, UV absorbing materials,and/or materials that are characterized by strong absorption peaks inthe near-IR or UV regions of the electromagnetic spectrum. Near-IRabsorbing materials include phthalocyanines, porphyrins,naphthalocyanines, squaraines, boron-dipyrromethenes, naphthalenes,rylenes, perylenes, tetracyano quinoidal thiophene compounds, tetracyanoindacene compounds, carbazole thiaporphyrin compounds, metaldithiolates, benzothiadiazole containing compounds, dicyanomethyleneindanone containing compounds, combinations thereof, and the like. UVabsorbing materials include fullerenes, rylenes, perylenes,benzimidazoles, hexacarbonitriles, triarylamines, bistriarylamines,phenanthrolines, para-phenylenes, combinations thereof, and the like.

FIGS. 2A-2E illustrate various example junction configurations forphotoactive layer 140. Photoactive layer 140 may optionally correspondto planar donor/acceptor configurations (as shown in FIG. 2A), mixeddonor/acceptor (bulk heterojunction) configurations (as shown in FIG.2B), planar and mixed donor/acceptor configurations (as shown in FIG.2C), gradient donor/acceptor configurations (as shown in FIG. 2D), orstacked heterojunction configurations (as shown in FIG. 2E).

Various materials may optionally be used as the photoactive layers 140,such as materials that absorb in the ultraviolet band or thenear-infrared band but that only absorb minimally, if at all, in thevisible band. In this way, the photoactive material may be used togenerate electron-hole pairs for powering an external circuit by way ofultraviolet and/or near-infrared absorption, leaving the visible lightrelatively unperturbed to provide visible transparency. As illustrated,photoactive layer 140 may comprise a planar heterojunction includingseparate donor and acceptor layers. Photoactive layer 140 mayalternatively comprise a planar-mixed heterojunction structure includingseparate acceptor and donor layers and a mixed donor-acceptor layer.Photoactive layer 140 may alternatively comprise a mixed heterojunctionstructure including a fully mixed acceptor-donor layer or thoseincluding a mixed donor-acceptor layer with various relativeconcentration gradients.

Photoactive layers may have any suitable thickness and may have anysuitable concentration or composition of photoactive materials toprovide a desired level of transparency and ultraviolet/near-infraredabsorption characteristics. Example thicknesses of a photoactive layermay range from about 1 nm to about 1 μm, about 1 nm to about 300 nm, orabout 1 nm to about 100 nm. In some cases, photoactive layers may bemade up of individual sub-layers or mixtures of layers to providesuitable photovoltaic power generation characteristics, as illustratedin FIGS. 2A-2E. The various configurations depicted in FIGS. 2A-2E maybe used and dependent on the particular donor and acceptor materialsused in order to provide advantageous photovoltaic power generation. Forexample, some donor and acceptor combinations may benefit fromparticular configurations, while other donor and acceptor combinationsmay benefit from other particular configurations. Donor materials andacceptor materials may be provided in any ratio or concentration toprovide suitable photovoltaic power generation characteristics. Formixed layers, the relative concentration of donors to acceptors isoptionally between about 20 to 1 and about 1 to 20. Optionally, therelative concentration of donors to acceptors is optionally betweenabout 5 to 1 and about 1 to 5. Optionally, donors and acceptors arepresent in a 1 to 1 ratio.

Various visibly transparent photoactive compounds are useful as anelectron donor photoactive material and, in some embodiments, may bepaired with suitable electron acceptor photoactive materials in order toprovide a useful photoactive layer in the photovoltaic device. Variousvisibly transparent photoactive compounds are useful as an electronacceptor photoactive material and may be paired with suitable electrondonor photoactive materials in order to provide a useful photoactivelayer in the photovoltaic device. Example donor and acceptor materialsare described in U.S. Provisional Application Nos. 62/521,154,62/521,158, 62/521,160, 62/521,211, 62/521,214, and 62/521,224, eachfiled on Jun. 16, 2017, which are hereby incorporated by reference intheir entireties.

In some embodiments, the chemical structure of various photoactivecompounds can be functionalized with one or more directing groups, suchas electron donating groups, electron withdrawing groups, orsubstitutions about or to a core metal atom, in order to providedesirable electrical characteristics to the material. For example, insome embodiments, the photoactive compounds are functionalized withamine groups, phenol groups, alkyl groups, phenyl groups, or otherelectron donating groups to improve the ability of the material tofunction as an electron donor in a photovoltaic device. As anotherexample, in some embodiments, the photoactive compounds arefunctionalized with cyano groups, halogens, sulfonyl groups, or otherelectron withdrawing groups to improve the ability of the material tofunction as an electron acceptor in a photovoltaic device.

In embodiments, the photoactive compounds are functionalized to providedesirable optical characteristics. For example, in some embodiments, thephotoactive compounds may be functionalized with an extended conjugationto redshift the absorption profile of the material. It will beappreciated that conjugation may refer to a delocalization of pielectrons in a molecule and may be characterized by alternating singleand multiple bonds in a molecular structure. For example,functionalizations that extend the electron conjugation may includefusing one or more aromatic groups to the molecular structure of thematerial. Other functionalizations that may provide extended conjugationinclude alkene functionalization, such as by a vinyl group, aromatic orheteroaromatic functionalization, carbonyl functionalization, such as byan acyl group, sulfonyl functionalization, nitro functionalization,cyano functionalization, etc. It will be appreciated that variousmolecular functionalizations may impact both the optical and theelectrical properties of the photoactive compounds.

It will be appreciated that device function may be impacted by themorphology of the active layers in the solid state. Separation ofelectron donors and acceptors into discrete domains, with dimensions onthe scale of the exciton diffusion length and large interfacial areas,can be advantageous for achieving high device efficiency.Advantageously, the molecular framework of the photoactive materials canbe tailored to control the morphology of the materials. For example, theintroduction of functional groups as described herein can have largeimpacts to the morphology of the material in the solid state, regardlessof whether such modifications impact the energetics or electronicproperties of the material. Such morphological variations can beobserved in pure materials and when a particular material is blendedwith a corresponding donor or acceptor. Useful functionalities tocontrol morphology include, but are not limited to, addition of alkylchains, conjugated linkers, fluorinated alkanes, bulky groups (e.g.,tert-butyl, phenyl, naphthyl or cyclohexyl), as well as more complexcoupling procedures designed to force parts of the structure out of theplane of the molecule to inhibit excessive crystallization.

In embodiments, other molecular structural characteristics may providedesirable electrical and optical properties in the photoactivecompounds. For example, in some embodiments, the photoactive compoundsmay exhibit portions of the molecule that may be characterized aselectron donating while other portions of the molecule may becharacterized as electron accepting. Without wishing to be bound by anytheory, molecules including alternating electron donating and electronaccepting portions may result in redshifting the absorptioncharacteristics of the molecule as compared to similar molecules lackingalternating electron donating and electron accepting portions. Forexample, alternating electron donating and electron accepting portionsmay decrease or otherwise result in a lower energy gap between a highestoccupied molecular orbital and a lowest unoccupied molecular orbital.Organic donor and/or acceptor groups may be useful as R-groupsubstituents, such as on any aryl, aromatic, heteroaryl, heteroaromatic,alkyl, or alkenyl group, in the visibly transparent photoactivecompounds.

When the donor/acceptor materials are incorporated as a photoactivelayer in a transparent photovoltaic device as either an electron donoror electron acceptor, the layer thicknesses can be controlled to varydevice output, absorbance, or transmittance. For example, increasing thedonor or acceptor layer thickness can increase the light absorption inthat layer. In some cases, increasing a concentration of donor/acceptormaterials in a donor or acceptor layer may similarly increase the lightabsorption in that layer. However, in some embodiments, a concentrationof donor/acceptor materials may not be adjustable, such as when activematerial layers comprise pure or substantially pure layers ofdonor/acceptor materials or pure or substantially pure mixtures ofdonor/acceptor materials. Optionally, donor/acceptor materials may beprovided in a solvent or suspended in a carrier, such as a buffer layermaterial, in which case the concentration of donor/acceptor materialsmay be adjusted. In some embodiments, the donor layer concentration isselected where the current produced is maximized. In some embodiments,the acceptor layer concentration is selected where the current producedis maximized.

However, the charge collection efficiency can decrease with increasingdonor or acceptor thickness due to the increased “travel distance” forthe charge carriers. Therefore, there may be a trade-off betweenincreased absorption and decreasing charge collection efficiency withincreasing layer thickness. It can thus be advantageous to selectmaterials that have a high absorption coefficient and/or concentrationto allow for increased light absorption per thickness. In someembodiments, the donor layer thickness is selected where the currentproduced is maximized. In some embodiments, the acceptor layer thicknessis selected where the current produced is maximized.

In addition to the individual photoactive layer thicknesses, thethickness and composition of the other layers in the transparentphotovoltaic device can also be selected to enhance absorption withinthe photoactive layers. The other layers (buffer layers, electrodes,etc.), are typically selected based on their optical properties (indexof refraction and extinction coefficient) in the context of the thinfilm device stack and resulting optical cavity. For example, anear-infrared absorbing photoactive layer can be positioned in the peakof the optical field for the near-infrared wavelengths where it absorbsto maximize absorption and resulting current produced by the device.This can be accomplished by spacing the photoactive layer at anappropriate distance from the electrode using a second photoactive layerand/or optical layers as spacer. A similar scheme can be used forultraviolet absorbing photoactive layers. In many cases, the peaks ofthe longer wavelength optical fields will be positioned further from themore reflective of the two transparent electrodes compared to the peaksof the shorter wavelength optical fields. Thus, when using separatedonor and acceptor photoactive layers, the donor and acceptor can beselected to position the more red-absorbing (longer wavelength) materialfurther from the more reflective electrode and the more blue absorbing(shorter wavelength) closer to the more reflective electrode.

In some embodiments, optical layers may be included to increase theintensity of the optical field at wavelengths where the donor absorbs inthe donor layer to increase light absorption and hence, increase thecurrent produced by the donor layer. In some embodiments, optical layersmay be included to increase the intensity of the optical field atwavelengths where the acceptor absorbs in the acceptor layer to increaselight absorption and hence, increase the current produced by theacceptor layer. In some embodiments, optical layers may be used toimprove the transparency of the stack by either decreasing visibleabsorption or visible reflection. Further, the electrode material andthickness may be selected to enhance absorption outside the visiblerange within the photoactive layers, while preferentially transmittinglight within the visible range.

Optionally, enhancing spectral coverage of a visibly transparentphotovoltaic device is achieved by the use of a multi-cell series stackof visibly transparent photovoltaic devices, referred to as tandemcells, which may be included as multiple stacked instances of bufferlayer 130, photoactive layer 140, and buffer layer 132, as describedwith reference to FIG. 1. This architecture includes more than onephotoactive layer, which are typically separated by a combination ofbuffer layer(s) and/or thin metal layers, for example. In thisarchitecture, the currents generated in each subcell flow in series tothe opposing electrodes and therefore, the net current in the cell islimited by the smallest current generated by a particular subcell, forexample. The open circuit voltage (V_(OC)) is equal to the sum of theV_(OC) values of the subcells. By combining sub-cells fabricated withdifferent donor-acceptors pairs which absorb in different regions of thesolar spectrum, a significant improvement in efficiency relative to asingle junction cell can be achieved.

FIG. 3 illustrates a simplified plot of the solar spectrum, human eyesensitivity, and exemplary visibly transparent photovoltaic deviceabsorption as a function of wavelength. As illustrated in FIG. 3,embodiments of the present invention utilize photovoltaic structuresthat have low absorption in the visible wavelength band between about450 nm and about 650 nm, but absorb in the UV and NIR bands, i.e.,outside the visible wavelength band, enabling visibly transparentphotovoltaic operation. The ultraviolet band or ultraviolet region maybe described, in embodiments, as wavelengths of light of between about200 nm and 450 nm. It will be appreciated that useful solar radiation atground level may have limited amounts of ultraviolet less than about 280nm and, thus, the ultraviolet band or ultraviolet region may bedescribed as wavelengths of light of between about 280 nm and 450 nm, insome embodiments. The near-infrared band or near-infrared region may bedescribed, in embodiments, as wavelengths of light of between about 650nm and 1400 nm. Various compositions described herein may exhibitabsorption including a UV peak and/or a NIR peak with a maximumabsorption strength in the visible region that is smaller than that inthe NIR region or UV region.

FIG. 4 illustrates a schematic energy level diagram overview foroperation of an example organic photovoltaic device, such as visiblytransparent photovoltaic device 100. For example, in such a photovoltaicdevice, various photoactive materials may exhibit electron donor orelectron acceptor characteristics, depending on their molecularproperties and the types of materials that are used for buffer layers,electrodes, etc. As depicted in FIG. 4, each of the donor and acceptormaterials has a HOMO and a LUMO. A transition of an electron from theHOMO to the LUMO may be imparted by absorption of photons. The energybetween the HOMO and the LUMO (the HOMO-LUMO gap) of a materialrepresents approximately the energy of the optical band gap of thematerial. For the electron donor and electron acceptor materials usefulwith the transparent photovoltaic devices provided herein, the HOMO-LUMOgap for the electron donor and electron acceptor materials mayoptionally fall outside the energy of photons in the visible range. Forexample, the HOMO-LUMO gap may be in the ultraviolet region or thenear-infrared region, depending on the photoactive materials. It will beappreciated that the HOMO is comparable to the valence band inconventional conductors or semiconductors, while the LUMO is comparableto the conduction band in conventional conductors or semiconductors.

The buffer layer adjacent to the donor, generally referred to as theanode buffer layer or hole transport layer, is selected such that HOMOlevel or valence band (in the case of inorganic materials) of the bufferlayer is aligned in the energy landscape with the HOMO level of thedonor to transport holes from the donor to the anode (transparentelectrode). In some embodiments, it may be useful for the buffer layerto have high hole mobility. The buffer layer adjacent to the acceptor,generally referred to as the cathode buffer layer or electron transportlayer, is selected such that LUMO level or conduction band (in the caseof inorganic materials) of the buffer layer is aligned in the energylandscape with the LUMO level of the acceptor to transport electronsfrom the acceptor to the cathode (transparent electrode). In someembodiments, it may be useful for the buffer layer to have high electronmobility.

FIG. 5A-5D illustrate various plots showing example absorption bands fordifferent electron donor and electron acceptor configurations usefulwith visibly transparent photovoltaic devices. In FIG. 5A, the donormaterial exhibits absorption in the NIR, while the acceptor materialexhibits absorption in the UV. FIG. 5B depicts the oppositeconfiguration, where the donor material exhibits absorption in the UV,while the acceptor material exhibits absorption in the NIR.

FIG. 5C depicts an additional configuration, where both the donor andacceptor materials exhibit absorption in the NIR. As illustrated in thefigures, the solar spectrum exhibits significant amounts of usefulradiation in the NIR with only relatively minor amounts in theultraviolet, making the configuration depicted in FIG. 5C useful forcapturing a large amount of energy from the solar spectrum. It will beappreciated that other embodiments are contemplated where both the donorand acceptor materials exhibit absorption in the NIR, such as depictedin FIG. 5D where the acceptor is blue shifted relative to the donor,opposite the configuration depicted in FIG. 5C, where the donor is blueshifted relative to the acceptor.

FIG. 6 illustrates a schematic energy level diagram of an exampleorganic photovoltaic device, according to some embodiments. The exampleorganic photovoltaic device includes an HTL, an electron donor material,and an electron acceptor material. The HTL is coupled to the electrondonor material and the electron donor material is coupled to theelectron acceptor material. As illustrated, the HOMO level of the HTL isdeeper than the HOMO level of the electron donor material such that anextraction barrier for holes is formed; photogenerated holes are unableto efficiently move from the electron donor material through the HTL tothe anode. Such an arrangement of materials can lead to a suppression ofphotocurrent from the photovoltaic device.

FIG. 7 illustrates a schematic energy level diagram of an exampleorganic photovoltaic device, according to some embodiments. The exampleorganic photovoltaic device is similar to the device of FIG. 6 exceptthat the HOMO level of the HTL is shallower than the HOMO level of theelectron donor material. As illustrated, holes are unable to efficientlyinject from the HTL into the electron donor material. Such anarrangement of materials can lead to a suppression of the built-inpotential and a reduction of the dark current passing through the deviceunder forward bias. This may result in s-kinked current-voltage curves,reduced fill factor (FF), and reduced open-circuit voltage (V_(oc)).

FIG. 8 illustrates a schematic energy level diagram of an exampleorganic photovoltaic device, according to some embodiments. The exampleorganic photovoltaic device includes an HTL, a metal-oxide interlayer,an electron donor material, and an electron acceptor material. The HTLis coupled to the metal-oxide interlayer, the metal-oxide interlayer iscoupled to the electron donor material, and the electron donor materialis coupled to the electron acceptor material. The metal-oxide interlayerprovides an ohmic connection between the adjacent HTL and electron donorlayers. As a result, there is neither an extraction nor an injectionbarrier at the interface of the HTL and electron donor material, as isillustrated in FIGS. 6 and 7. The combination of the HTL and themetal-oxide interlayer may be referred to as a compound HTL.

As illustrated, for a first HOMO level 802 of the HTL that is equal tothe HOMO level of the electron donor material, holes are able to movebarrier-free from the HTL through the electron donor material as well asfrom the electron donor material through the HTL. Similarly, for adifferent HTL material with a second HOMO level 804 that is higher thanthe HOMO level of the electron donor material, holes are again able tomove barrier-free from the HTL through the electron donor material aswell as from the electron donor material through the HTL. Similarly, fora different HTL material with a third HOMO level 806 that is lower thanthe HOMO level of the electron donor material, holes are again able tomove barrier-free from the HTL through the electron donor material aswell as from the electron donor material through the HTL. Due to theohmic contact at the HTL/metal-oxide and metal-oxide/donor interfaces,the presence of the metal-oxide interlayer decouples the HOMO level ofthe HTL from the HOMO level of the electron donor material and thereforeenables a wide range of HTL materials with different energy levels to beused.

FIG. 9 illustrates a device structure 900 for an example organicphotovoltaic device having a single compound HTL 950, according to someembodiments of the present invention. Device structure 900 may include asubstrate 902, an anode 904 disposed above and/or formed onto substrate902, an organic HTL 906 disposed above and/or formed onto anode 904, ametal-oxide interlayer 908 disposed above and/or formed onto organic HTL906, one or more active layers 910 disposed above and/or formed ontometal-oxide interlayer 908, and a cathode 912 disposed above and/orformed onto active layers 910.

During operation, device structure 900 receives light that passesthrough cathode 912 and/or anode 904 and is partially absorbed by activelayers 910 (e.g., the UV and/or NIR wavelengths of the received light).The absorbed light causes a separation of charge carriers of oppositetypes in active layers 910, with electrons moving from active layers 910upward through cathode 912 and holes moving from active layers 910downward through metal-oxide interlayer 908, organic HTL 906, and anode904. As described in reference to FIG. 8, any misalignment between theHOMO levels of active layers 910 and organic HTL 906 that wouldotherwise prevent movement of holes downward from active layers 910through organic HTL 906 is mitigated by the presence of metal-oxideinterlayer 908.

FIG. 10 illustrates a device structure 1000 for an example organicphotovoltaic device having a multilayer compound HTL 1050, according tosome embodiments of the present invention. Device structure 1000 mayinclude a substrate 1002, an anode 1004 disposed above and/or formedonto substrate 1002, a first organic HTL 1006-1 disposed above and/orformed onto anode 1004, a first metal-oxide interlayer 1008-1 disposedabove and/or formed onto organic HTL 1006-1, a second organic HTL 1006-2disposed above and/or formed onto metal-oxide interlayer 1008-1, asecond metal-oxide interlayer 1008-2 disposed above and/or formed ontoorganic HTL 1006-2, one or more active layers 1010 disposed above and/orformed onto metal-oxide interlayer 1008-2, and a cathode 1012 disposedabove and/or formed onto active layers 1010.

Device structure 1000 differs from device structure 900 by including asecond compound HTL comprising organic HTL 1006-2 and metal-oxideinterlayer 1008-2, which can provide additional decoupling of HOMOlevels. During operation, device structure 1000 receives light thatpasses through cathode 1012 and/or anode 1004 and is partially absorbedby active layers 1010 (e.g., the UV and/or NIR wavelengths of thereceived light). The absorbed light causes a separation of chargecarriers of opposite types in active layers 1010, with electrons movingfrom active layers 1010 upward through cathode 1012 and holes movingfrom active layers 1010 downward through metal-oxide interlayer 1008-2,organic HTL 1006-2, metal-oxide interlayer 1008-1, organic HTL 1006-1,and anode 1004. As described in reference to FIG. 8, any misalignmentbetween the HOMO levels of active layers 1010 and organic HTLs 1006 thatwould otherwise prevent movement of holes downward from active layers1010 through organic HTLs 1006 is mitigated by the presence ofmetal-oxide interlayers 1008.

FIG. 11 illustrates a device structure 1100 for an example organicphotovoltaic device having a single compound ETL 1150, according to someembodiments of the present invention. Device structure 1100 may includea substrate 1102, an anode 1104 disposed above and/or formed ontosubstrate 1102, one or more active layers 1106 disposed above and/orformed onto anode 1104, a metal-oxide interlayer 1108 disposed aboveand/or formed onto active layers 1106, an organic ETL 1110 disposedabove and/or formed onto metal-oxide interlayer 1108, and a cathode 1112disposed above and/or formed onto organic ETL 1110.

During operation, device structure 1100 receives light that passesthrough cathode 1112 and/or anode 1104 and is partially absorbed byactive layers 1106 (e.g., the UV and/or NIR wavelengths of the receivedlight). The absorbed light causes a separation of charge carriers ofopposite types in active layers 1106, with electrons moving from activelayers 1106 upward through metal-oxide interlayer 1108, organic ETL1110, and cathode 1112 and holes moving from active layers 1106 downwardthrough anode 1104. Similar to that described in reference to FIG. 8,any misalignment between the LUMO levels of active layers 1106 andorganic ETL 1110 that would otherwise prevent movement of electronsupward from active layers 1106 through organic ETL 1110 is mitigated bythe presence of metal-oxide interlayer 1108.

FIG. 12 illustrates a device structure 1200 for an example organicphotovoltaic device having a multilayer compound ETL 1250, according tosome embodiments of the present invention. Device structure 1200 mayinclude a substrate 1202, an anode 1204 disposed above and/or formedonto substrate 1202, one or more active layers 1206 disposed aboveand/or formed onto anode 1204, a first metal-oxide interlayer 1208-1disposed above and/or formed onto active layers 1206, a first organicETL 1210-1 disposed above and/or formed onto metal-oxide interlayer1208-1, a second metal-oxide interlayer 1208-2 disposed above and/orformed onto organic ETL 1210-1, a second organic ETL 1210-2 disposedabove and/or formed onto metal-oxide interlayer 1208-2, and a cathode1212 disposed above and/or formed onto organic ETL 1210-2.

During operation, device structure 1200 receives light that passesthrough cathode 1212 and/or anode 1204 and is partially absorbed byactive layers 1206 (e.g., the UV and/or NIR wavelengths of the receivedlight). The absorbed light causes a separation of charge carriers ofopposite types in active layers 1206, with electrons moving from activelayers 1206 upward through metal-oxide interlayer 1208-1, organic ETL1210-1, metal-oxide interlayer 1208-2, organic ETL 1210-2, and cathode1212 and holes moving from active layers 1206 downward through anode1204. Similar to that described in reference to FIG. 8, any misalignmentbetween the LUMO levels of active layers 1206 and organic ETLs 1210 thatwould otherwise prevent movement of electrons upward from active layers1206 through organic ETLs 1210 is mitigated by the presence ofmetal-oxide interlayer 1208.

FIG. 13 illustrates a device structure 1300 for an example organicphotovoltaic device having a single compound HTL 1302, according to someembodiments of the present invention. Device structure 1300 maycorrespond to an example implementation of device structure 900described in reference to FIG. 9 that includes a single compound HTL.Accordingly, operation of device structure 1300 may be similar to thatdescribed in reference to device structure 900. Device structure 1300includes a glass substrate (corresponding to substrate 902), an ITOlayer (corresponding to anode 904), a MoO₃ hole-injection layer (HIL)(corresponding to a metal-oxide layer), an HTL (corresponding to organicHTL 906), a MoO₃ IL (corresponding to metal-oxide interlayer 908), ap-6P layer (corresponding to a p-phenylene layer), a TAPC:C₇₀ blend(corresponding to active layers 910) and a TPBi:C₆₀ blend (correspondingto an organic buffer layer), an Ag layer (corresponding to cathode 912),and a HAT-CN layer (corresponding to an optical anti-reflection layer).

FIG. 14A-14D illustrate a selection of various materials that can beutilized in the HTL of device structure 1300. The HTL may also include,but is not limited to, materials such as those listed in reference tobuffer layers 130 and 132. FIG. 14A-14C illustrates an F₈ZnPc, anF₁₆CuPc molecule, and a ClAlPc molecule of the phthalocyanine class oforganic compounds, respectively. FIG. 14D illustrates a TAPC molecule ofthe triarylamine class of compounds.

FIG. 15 illustrates energy levels associated with F₈ZnPc, F₁₆CuPc,ClAlPc, and TAPC materials used as HTLs according to some embodiments ofthis invention. As shown in FIG. 15, the HOMO levels of the materialsspan approximately 1.6 eV (−6.8 eV to −5.2 eV), thus providing a widerange of energy levels to be used in the HTL.

FIG. 16A-16D illustrate plots showing the power conversion efficiency(PCE) as a function of metal-oxide interlayer thickness for variousorganic photovoltaic devices. FIG. 16A illustrates a plot showing PCE asa function of metal-oxide interlayer thickness of device structure 1300with a F₈ZnPc material used as the HTL. The solid line corresponds tothe HTL having only the F₈ZnPc material and the dashed line correspondsto the HTL having a 20 vol % doping of the F₈ZnPc material with MoO₃. Asshown in FIG. 16A, improvements can be achieved to the PCE by increasingthe interlayer thickness (for low interlayer thicknesses) and/or bydoping the HTL with MoO₃.

Improvements to the PCE between interlayer thicknesses of 0 nm and 2 nmcan be attributed to the decoupling effect of the HOMO levels of theF₈ZnPc material and the TAPC:C₇₀ blend provided by the MoO₃ IL. Whileincreasing the interlayer thickness from 0 nm to 2 nm and then again to4 nm was found to increase the PCE for both the doped and undoped HTL,the PCE for the doped HTL without an interlayer (thickness equal to 0nm) was found to be greater than the PCE for the undoped HTL at allinterlayer thicknesses.

FIG. 16B illustrates a plot showing PCE as a function of metal-oxideinterlayer thickness of device structure 1300 with a F₁₆CuPc materialused as the HTL. The solid line corresponds to the HTL having only theF₁₆CuPc material and the dashed line corresponds to the HTL having a 20vol % doping of the F₁₆CuPc material with MoO₃. Similar to FIG. 16A,FIG. 16B demonstrates improvements to the PCE by increasing theinterlayer thickness (for low interlayer thicknesses) and/or by dopingthe HTL with MoO₃. For the undoped HTL, the improvement in the PCE wasfound to be significant when increasing the interlayer thickness from 0nm to 2 nm, with small improvements thereafter from 2 nm to 6 nm. Thisimprovement can be attributed to the decoupling effect of the HOMOlevels of the F₁₆CuPc material and the TAPC:C₇₀ blend provided by theMoO₃ IL. In contrast, for the doped HTL, the improvements in the PCEwere less significant yet were still observed when increasing theinterlayer thickness from 0 nm to 6 nm.

FIG. 16C illustrates a plot showing PCE as a function of metal-oxideinterlayer thickness of device structure 1300 with a ClAlPc materialused as the HTL. The solid line corresponds to the HTL having only theClAlPc material and the dashed line corresponds to the HTL having a 20vol % doping of ClAlPc with MoO₃. In contrast to FIGS. 16A and 16B, FIG.16C demonstrates that for some materials extrinsic doping of the HTL maynot provide significant improvements to the PCE beyond that provided byincreasing the interlayer thickness.

Improvements to the PCE between interlayer thicknesses of 0 nm and 2 nmcan be attributed to the decoupling effect of the HOMO levels of theClAlPc material and the TAPC:C₇₀ blend provided by the MoO₃ IL. Notably,between an interlayer thickness of 2 nm and 4 nm, the undoped HTLoutperformed the doped HTL, while increasing the interlayer thicknessbeyond 4 nm caused the PCE to decrease for the undoped HTL.

FIG. 16D illustrates a plot showing PCE as a function of metal-oxideinterlayer thickness of device structure 1300 with a TAPC material usedas the HTL. The solid line corresponds to the HTL having only the TAPCmaterial and the dashed line corresponds to the HTL having a 5 vol %doping of the TAPC material with MoO₃. Similar to FIGS. 16A and 16B,FIG. 16D demonstrates significant improvements to the PCE by increasingthe interlayer thickness and/or by doping the HTL with MoO₃.

Notably, for the undoped HTL, steady and roughly linear improvements tothe PCE are observed by increasing the interlayer thickness from 0 nm to2 nm, from 2 nm to 4 nm, and from 4 nm to 6 nm. For the doped HTL, thegreatest gains in the PCE were observed between interlayer thickness of0 nm and 2 nm, with less significant improvements thereafter. Similar toFIG. 16A, the PCE for the doped HTL without an interlayer (thicknessequal to 0 nm) was found to be greater than the PCE for the undoped HTLat all interlayer thicknesses.

FIGS. 16A-16D illustrate the generality of the compound HTL concept. Itcan be applied to HTL materials that require extrinsic doping forefficient charge transport (FIG. 16D) as well as materials that maysufficiently transport charge but lack proper energy level alignmentwith the active layers (FIGS. 16A-C). The approach works across a widerange of HTL compounds regardless of their energy levels, as these aredecoupled from the energy levels of the active layers. For example, nopower generation is achieved with F16CuPc without either extrinsicdoping and/or MoO3 interlayer incorporation (FIG. 16B). In cases whereextrinsic doping is used, the compound HTL approach further improvesperformance.

FIG. 17 illustrates a device structure 1700 for an example organicphotovoltaic device having a single compound HTL 1702, according to someembodiments of the present invention. Device structure 1700 maycorrespond to an example implementation of device structure 900described in reference to FIG. 9 that includes a single compound HTLcomprising a variable F₈ZnPc HTL (with a thickness between 5 and 25 nm)and a metal-oxide interlayer. Accordingly, operation of device structure1700 may be similar to that described in reference to device structure900.

Device structure 1700 includes a glass substrate (corresponding tosubstrate 902), an ITO layer (corresponding to anode 904), a MoO₃ HIL(corresponding to a metal-oxide hole-injection layer), a F₈ZnPc HTL(corresponding to organic HTL 906), a MoO₃ IL (corresponding tometal-oxide interlayer 908), a p-6P layer (corresponding to ap-phenylene buffer layer), a TAPC:C₇₀ blend (corresponding to activelayers 910) and a TPBi:C₆₀ blend (corresponding to an organic bufferlayer), an Ag layer (corresponding to cathode 912), and a HAT-CN layer(corresponding to an optical anti-reflection layer).

FIG. 18A-18C illustrate plots showing experimental values acquired foran organic photovoltaic device having device structure 1700. FIG. 18Aillustrates a plot showing PCE as a function of HTL thickness. Thedashed line corresponds to device structure 1700 including a metal-oxideinterlayer with a thickness of 6 nm and the solid line corresponds todevice structure 1700 without a metal-oxide interlayer. As shown in FIG.18A, inclusion of the metal-oxide interlayer causes an immediateimprovement to the PCE across all measured HTL thicknesses, and greatergains in the measured PCEs are achieved at greater thicknesses. Due to aprogressive increase in electrical resistance, PCE decreases withincreasing HTL thickness beyond 5 nm. However, the inclusion of themetal-oxide interlayer provides a significant reduction in resistancefor all thicknesses due to contact doping at the interface with the HTL,allowing for thicker HTLs to be used without extrinsic doping.

FIG. 18B illustrates a plot showing transmitted color parameters a* andb* as a function of HTL thickness. The dashed lines correspond to devicestructure 1700 including a metal-oxide interlayer with a thickness of 6nm and the solid lines correspond to device structure 1700 without ametal-oxide interlayer. FIG. 18C illustrates a plot showing reflectedcolor parameters a* and b* as a function of HTL thickness. The dashedlines corresponds to device structure 1700 including a metal-oxideinterlayer with a thickness of 6 nm and the solid lines correspond todevice structure 1700 without a metal-oxide interlayer. FIG. 18Cdemonstrates the significant effect that the thickness of the HTLmaterial can have on the reflected color. For device structure 1700employing a metal-oxide interlayer, the color can be tuned across a widerange with minimal PCE loss.

FIG. 19A-19C illustrate device structures 1900 for example organicphotovoltaic devices having compound HTLs 1902, according to someembodiments of the present invention. FIG. 19A illustrates a devicestructure 1900A for an example organic photovoltaic device having asingle compound HTL 1902A. Device structure 1900A may correspond to anexample implementation of device structure 900 described in reference toFIG. 9. Accordingly, operation of device structure 1900A may be similarto that described in reference to device structure 900. Device structure1900A includes a glass substrate (corresponding to substrate 902), anITO layer (corresponding to anode 904), a MoO₃ HIL (corresponding to ametal-oxide hole-injection layer), a F₈ZnPc HTL (corresponding toorganic HTL 906), a MoO₃ IL (corresponding to metal-oxide interlayer908), a p-6P layer, a TAPC:C₇₀ blend (corresponding to active layers910) and a TPBi:C₆₀ blend (corresponding to an organic buffer layer), anAg layer (corresponding to cathode 912), and a HAT-CN layer.

FIG. 19B illustrates a device structure 1900B for an example organicphotovoltaic device having a multilayer compound HTL 1902B. Devicestructure 1900B may correspond to an example implementation of devicestructure 1000 described in reference to FIG. 10. Accordingly, operationof device structure 1900B may be similar to that described in referenceto device structure 1000. Device structure 1900B includes a glasssubstrate (corresponding to substrate 1002), an ITO layer (correspondingto anode 1004), a MoO₃ HIL (corresponding to a metal-oxidehole-injection layer), a first F₈ZnPc HTL (corresponding to organic HTL1006-1), a first MoO₃ IL (corresponding to metal-oxide interlayer1008-1), a second F₈ZnPc HTL (corresponding to organic HTL 1006-2), asecond MoO₃ IL (corresponding to metal-oxide interlayer 1008-2), a p-6Player, a TAPC:C₇₀ blend (corresponding to active layers 1010) and aTPBi:C₆₀ blend (corresponding to an organic buffer layer), an Ag layer(corresponding to cathode 1012), and a HAT-CN layer (corresponding to anoptical anti-reflection layer).

FIG. 19C illustrates a device structure 1900C for an example organicphotovoltaic device having a multilayer compound HTL 1902C. Devicestructure 1900C may correspond to an extension of device structure 1000described in reference to FIG. 10, and may include a glass substrate(corresponding to substrate 1002), an ITO layer (corresponding to anode1004), a MoO₃ HIL (corresponding to a metal-oxide hole-injection layer),a first F₈ZnPc HTL (corresponding to organic HTL 1006-1), a first MoO₃IL (corresponding to metal-oxide interlayer 1008-1), a second F₈ZnPc HTL(corresponding to organic HTL 1006-2), a second MoO₃ IL (correspondingto metal-oxide interlayer 1008-2), a third F₈ZnPc HTL, a third MoO₃ IL,a fourth F₈ZnPc HTL, a fourth MoO₃ IL, a p-6P layer, a TAPC:C₇₀ blend(corresponding to active layers 1010) and a TPBi:C₆₀ blend(corresponding to an organic buffer layer), an Ag layer (correspondingto cathode 1012), and a HAT-CN layer (corresponding to an opticalanti-reflection layer).

FIG. 20 illustrates a plot showing the PCE as a function of the top-mostmetal-oxide interlayer thickness of device structures 1900. The solidline corresponds to device structure 1900A, the dot-dashed linecorresponds to device structure 1900B, and the dashed line correspondsto device structure 1900C. FIG. 20 demonstrates that furtherimprovements in the PCE can be achieved by dividing the HTL into smallerand smaller slices separated by metal-oxide interlayers. This furtherhighlights the contact doping effect of the HTL(s) provided by themetal-oxide interlayer(s). Additionally, by splitting a single compoundHTL (device structure 900) into multiple compound HTLs (device structure1000), multiple HTL compounds can be used in the same device structure.This provides a large degree of flexibility for engineering color andabsorption of the photovoltaic device.

FIG. 21 illustrates a device structure 2100 for an example organicphotovoltaic device having a multilayer compound HTL 2102 comprised oftwo different HTL materials, according to some embodiments of thepresent invention. Device structure 2100 may correspond to an exampleimplementation of device structure 1000 described in reference to FIG.10. Accordingly, operation of device structure 2100 may be similar tothat described in reference to device structure 1000. Device structure2100 includes a glass substrate (corresponding to substrate 1002), anITO layer (corresponding to anode 1004), a MoO₃ HIL (corresponding to ametal-oxide hole-injection layer), a TAPC:MoO₃ HTL (corresponding toorganic HTL 1006-1), a first MoO₃ IL (corresponding to metal-oxideinterlayer 1008-1), a ClAlPc HTL (corresponding to organic HTL 1006-2),a second MoO₃ IL (corresponding to metal-oxide interlayer 1008-2), ap-6P layer, a TAPC:C₇₀ blend and C₆₀ layer (corresponding to activelayers 1010) and a TPBi:C₆₀ blend (corresponding to an organic bufferlayer), an Ag layer (corresponding to cathode 1012), and a HAT-CN layer(corresponding to an optical anti-reflection layer).

FIG. 22A-22C illustrate plots showing experimental values acquired foran organic photovoltaic device having device structure 2100. FIG. 22Aillustrates a plot showing PCE as a function of thickness of theTAPC:MoO₃ HTL. FIG. 22B illustrates a plot showing transmitted colorparameters a* and b* as a function of thickness of the TAPC:MoO₃ HTL.FIG. 22C illustrates a plot showing reflected color parameters a* and b*as a function of thickness of the TAPC:MoO₃ HTL. FIG. 22C demonstratesthe significant effect that the thickness of one of the HTLs can have onthe reflected color, which can be engineered independently of thetransmitted color and PCE.

FIG. 23 illustrates a device structure 2300 for an example organicphotovoltaic device having a single compound HTL 2302, according to someembodiments of the present invention. Device structure 2300 maycorrespond to an example implementation of device structure 900described in reference to FIG. 9 that includes a single compound HTLcomprising a variable p-5P HTL (with a thickness between 0 and 40 nm)and a metal-oxide interlayer. Accordingly, operation of device structure2300 may be similar to that described in reference to device structure900. Device structure 2300 includes a glass substrate (corresponding tosubstrate 902), an ITO layer (corresponding to anode 904), a MoO₃ HIL(corresponding to a metal-oxide hole-injection layer), a p-5P HTL(corresponding to organic HTL 906), a MoO₃ IL (corresponding tometal-oxide interlayer 908), a DTDCPB:C₇₀ blend, a DTDCTB:C₇₀ blend, anda C₆₀ layer (corresponding to active layers 910), a TPBi:C₆₀ layer(corresponding to an organic buffer layer), an Ag layer (correspondingto cathode 912), and a HAT-CN layer (corresponding to an opticalanti-reflection layer).

FIG. 24A-24C illustrate plots showing experimental values acquired foran organic photovoltaic device having device structure 2300. FIG. 24Aillustrates a plot showing PCE as a function of thickness of the p-5PHTL. FIG. 24B illustrates a plot showing transmitted color parameters a*and b* as a function of thickness of the p-5P HTL. FIG. 24C illustratesa plot showing reflected color parameters a* and b* as a function ofthickness of the p-5P HTL. By using a transparent HTL such as p-5P inthe compound HTL, the reflected color can be engineered completelyindependently of either transmitted color or PCE.

FIG. 25 illustrates a device structure 2500 for an example organicphotovoltaic device having a single compound HTL 2550 in an invertedarchitecture, according to some embodiments of the present invention.Device structure 2500 may include a substrate 2502, a cathode 2504disposed above and/or formed onto substrate 2502, one or more activelayers 2506 disposed above and/or formed onto cathode 2504, ametal-oxide interlayer 2508 disposed above and/or formed onto activelayers 2506, an organic HTL 2510 disposed above and/or formed ontometal-oxide interlayer 2508, and an anode 2512 disposed above and/orformed onto organic HTL 2510.

During operation, device structure 2500 receives light that passesthrough anode 2512 and/or cathode 2502 and is partially absorbed byactive layers 2506 (e.g., the UV and/or NIR wavelengths of the receivedlight). The absorbed light causes a separation of charge carriers ofopposite types in active layers 2506, with electrons moving from activelayers 2506 downward through cathode 2504 and holes moving upwardthrough metal-oxide interlayer 2508, organic HTL 2510, and anode 2512.

In several of the previous examples, the cells were designed such thatthe anode is deposited first, closest to the substrate, followed by theactive layers, followed by the cathode. However, as shown by theillustrated example in FIG. 25, it is also possible to reverse the ordersuch that the cathode is nearest the substrate, followed by the activelayers and then the anode. Device structure 2500 may be referred to asan “inverted architecture”. Under operation, electrons still flow to thecathode and holes still flow to the anode. In such an architecture, theHTL is deposited atop the active layers prior to deposition of the topelectrode. A compound HTL can be used in an inverted architecture aswell, as shown in device structure 2500.

FIG. 26 illustrates a device structure 2600 for an example organicphotovoltaic device having a single compound ETL 2650 in an invertedarchitecture, according to some embodiments of the present invention.Device structure 2600 may include a substrate 2602, a cathode 2604disposed above and/or formed onto substrate 2602, an organic ETL 2606disposed above and/or formed onto cathode 2604, a metal-oxide interlayer2608 disposed above and/or formed onto organic ETL 2606, one or moreactive layers 2610 disposed above and/or formed onto metal-oxideinterlayer 2608, and an anode 2612 disposed above and/or formed ontoactive layers 2610.

During operation, device structure 2600 receives light that passesthrough anode 2612 and/or cathode 2604 and is partially absorbed byactive layers 2610 (e.g., the UV and/or NIR wavelengths of the receivedlight). The absorbed light causes a separation of charge carriers ofopposite types in active layers 2610, with electrons moving from activelayers 2610 downward through metal-oxide interlayer 2608, organic ETL2606, and cathode 2604 and holes moving from active layers 2610 upwardthrough anode 2612.

FIG. 27 illustrates a device structure 2700 for an example organicphotovoltaic device having a single compound HTL 2702 in an invertedarchitecture, according to some embodiments of the present invention.Device structure 2700 may correspond to an example implementation ofdevice structure 2500 described in reference to FIG. 25 that includes asingle compound HTL. Accordingly, operation of device structure 2700 maybe similar to that described in reference to device structure 2500.

Device structure 2700 includes a glass substrate (corresponding tosubstrate 2502), an ITO layer (corresponding to cathode 2504), aTPBi:C₆₀ blend (corresponding to an ETL), a C₆₀ layer and a TAPC:C₇₀blend (corresponding to active layers 2506), a MoO₃ IL (corresponding tometal-oxide interlayer 2508), a ClAlPC:MoO₃ blend (corresponding toorganic HTL 2510), a MoO₃ HIL (corresponding to a metal-oxide layer), anAg layer (corresponding to anode 2512), and a HAT-CN layer(corresponding to both a buffer layer and an optical anti-reflectionlayer).

FIGS. 28A and 28B illustrate plots showing experimental values acquiredfor an organic photovoltaic device having device structure 2700. FIG.28A illustrates a plot showing PCE as a function of HTL thickness. Asshown in FIG. 28A, increasing the thickness of the compound HTL leads toan improvement in power conversion efficiency. FIG. 28B illustrates aplot showing transmitted color parameters a* and b* as a function of HTLthickness. The dashed line corresponds to transmitted color parameter a*and the solid line corresponds to transmitted color parameter b*. Whenilluminated with white light, the transmitted color of the devicechanges significantly with ClAlPC thickness due to the selectiveabsorption of that material. The transmission of the semitransparentdevice shows a significant decrease in transmitted a* and b* leading toa more neutral transmission. Such flexibility in color without loss inperformance is desirable in designing transparent photovoltaics.

FIG. 29 illustrates a device structure 2900 for an example organicphotovoltaic device having both a compound HTL 2950 and a compound ETL2952 in a conventional (or non-inverted) architecture, according to someembodiments of the present invention. Device structure 2900 may includea substrate 2902, an anode 2904 disposed above and/or formed ontosubstrate 2902, an organic HTL 2906 disposed above and/or formed ontoanode 2904, a metal-oxide interlayer 2908 disposed above and/or formedonto organic HTL 2906, one or more active layers 2910 disposed aboveand/or formed onto metal-oxide interlayer 2908, a metal-oxide interlayer2912 disposed above and/or formed onto active layers 2910, an organicETL disposed above and/or formed onto metal-oxide interlayer 2912, and acathode 2916 disposed above and/or formed onto organic ETL 2914.

FIG. 30 illustrates a device structure 3000 for an example organicphotovoltaic device having both a compound HTL 3050 and a compound ETL3052 in an inverted architecture, according to some embodiments of thepresent invention. Device structure 3000 may include a substrate 3002, acathode 3004 disposed above and/or formed onto substrate 3002, anorganic ETL 3006 disposed above and/or formed onto cathode 3004, ametal-oxide interlayer 3008 disposed above and/or formed onto organicETL 3006, one or more active layers 3010 disposed above and/or formedonto metal-oxide interlayer 3008, a metal-oxide interlayer 3012 disposedabove and/or formed onto active layers 3010, an organic HTL disposedabove and/or formed onto metal-oxide interlayer 3012, and an anode 3016disposed above and/or formed onto organic HTL 3014.

FIG. 31 illustrates a method 3100 for making a photovoltaic device, suchas visibly transparent photovoltaic device 100, device structures 900,1000, 1100, 1200, 1300, 1700, 1900, 2100, 2300, 2500, 2600, 2700, 2900,3000, or any combinations thereof. In various embodiments, thephotovoltaic device may be visibly transparent or may be non-visiblytransparent or opaque. For example, any of the components of thephotovoltaic device described in reference to method 3100 may be visiblytransparent or non-visibly transparent or opaque. Furthermore, any ofthe components described in reference to method 3100 as being visiblytransparent may, in some embodiments, be non-visibly transparent oropaque. Method 3100 may include additional or fewer steps than isillustrated in FIG. 31. Furthermore, one or more steps of method 3100may be performed in a different order than is illustrated in FIG. 31.

Method 3100 begins at block 3102, where a substrate is provided, suchas, e.g., a transparent substrate. It will be appreciated that usefultransparent substrates include visibly transparent substrates, such asglass, plastic, quartz, and the like. Flexible and rigid substrates areuseful with various embodiments. Optionally, the transparent substrateis provided with one or more optical layers preformed on top and/orbottom surfaces.

At block 3104, one or more optical layers are optionally formed on orover the transparent substrate, such as on top and/or bottom surfaces ofthe transparent substrate. Optionally, the one or more optical layersare formed on other materials, such as an intervening layer or material,such as a transparent conductor. Optionally, the one or more opticallayers are positioned adjacent to and/or in contact with the visiblytransparent substrate. It will be appreciated that formation of opticallayers is optional, and some embodiments may not include optical layersadjacent to and/or in contact with the transparent substrate. Opticallayers may be formed using a variety of methods including, but notlimited to, one or more chemical deposition methods, such as plating,chemical solution deposition, spin coating, dip coating, slot-diecoating, blade coating, spray coating, chemical vapor deposition, plasmaenhanced chemical vapor deposition, and atomic layer deposition, or oneor more physical deposition methods, such as thermal evaporation,electron beam evaporation, molecular beam epitaxy, sputtering, pulsedlaser deposition, ion beam deposition, and electrospray deposition. Itwill be appreciated that useful optical layers include visiblytransparent optical layers. Useful optical layers include those thatprovide one or more optical properties including, for example,antireflection properties, wavelength selective reflection ordistributed Bragg reflection properties, index matching properties,encapsulation, or the like. Useful optical layers may optionally includeoptical layers that are transparent to ultraviolet and/or near-infraredlight. Depending on the configuration, however, some optical layers mayoptionally provide passive infrared and/or ultraviolet absorption.Optionally, an optical layer may include a visibly transparentphotoactive compound described herein.

At block 3106, a first (e.g., bottom) electrode is formed, such as,e.g., a first transparent electrode. As described above, the transparentelectrode may correspond to an ITO thin film or other transparentconducting film, such as thin metal films (e.g., Ag, Cu, etc.),multilayer stacks comprising thin metal films (e.g., Ag, Cu, etc.) anddielectric materials, or conductive organic materials (e.g., conductingpolymers, etc.). It will be appreciated that transparent electrodesinclude visibly transparent electrodes. Transparent electrodes may beformed using one or more deposition processes, including vacuumdeposition techniques, such as atomic layer deposition, chemical vapordeposition, physical vapor deposition, thermal evaporation, sputterdeposition, epitaxy, etc. Solution based deposition techniques, such asspin-coating, may also be used in some cases. In addition, transparentelectrodes may be patterned by way of microfabrication techniques, suchas lithography, lift off, etching, etc.

At block 3108, a charge-injection layer is formed. The charge-injectionlayer may be coupled to the first electrode. The charge-injection layermay be a hole-injection layer or an electron-injection layer. Thecharge-injection layer may be a metal-oxide hole-injection layer or ametal-oxide electron-injection layer. In some embodiments, the chargeinjection layer is a MoO₃ layer having a thickness of approximately 8nm.

At block 3110, a compound charge transport layer is formed. The compoundcharge transport layer may be a compound hole transport layer or acompound electron transport layer. In some embodiments, forming thecompound charge transport layer may include forming a charge transportlayer (block 3112) and forming a metal-oxide interlayer (block 3114).The charge transport layer may be a hold transport layer or an electrontransport layer. The charge transport layer may be coupled to the chargeinjection layer and the metal-oxide interlayer may be coupled to thecharge transport layer.

In some implementations, the compound charge transport layer formed atblock 3110 is a hole transport layer comprising of one or more of:F₈ZnPc, F₁₆CuPc, ClAlPc, or TAPC, and the metal-oxide interlayer iscomprised of one or more of: MoO₃, NiO, WO₃, V₂O₅, or ITO. In suchimplementations, the hole transport layer may have a thickness betweenapproximately 5 nm and 25 nm and the metal-oxide interlayer may have athickness between approximately 2 nm and 8 nm. In some implementations,the compound charge transport layer is an electron transport layercomprising of one or more of ZnO, AZO, FTO, SnO₂, Al:MoO₃, or TiO₂, andthe metal-oxide interlayer is comprised of one or more of: MoO₃, NiO,WO₃, V₂O₅, or ITO. In such implementations, the electron transport layermay have a thickness between approximately 5 nm and 25 nm and themetal-oxide interlayer may have a thickness between approximately 2 nmand 8 nm.

At block 3116, one or more photoactive layers are formed, such as on abuffer layer (formed by blocks 3108-3114) or on a transparent electrode(formed by block 3106). The photoactive layers may comprise electronacceptor layers and electron donor layers or co-deposited layers ofelectron donors and acceptors. Photoactive layers may be formed using avariety of methods including, but not limited to, one or more chemicaldeposition methods, such as a plating, chemical solution deposition,spin coating, dip coating, chemical vapor deposition, plasma enhancedchemical vapor deposition, and atomic layer deposition, or one or morephysical deposition methods, such as thermal evaporation, electron beamevaporation, molecular beam epitaxy, sputtering, pulsed laserdeposition, ion beam deposition, and electrospray deposition.

In some embodiments, block 3116 may include forming one or more BHJactive layers. For example, a first BHJ active layer may be formedcomprising a blend of a first electron donor material and a firstelectron acceptor material and a second BHJ active layer may be formedcomprising a blend of a second electron donor material and a secondelectron acceptor material. The first or second BHJ active layers may bea binary, ternary, quaternary, or a higher-order blend of electron donormaterials and electron acceptor materials. In some embodiments, thefirst BHJ active layer may have a distinct electron donor material fromthe second BHJ active layer (e.g., the first electron donor material maybe different than the second electron donor material). In someembodiments, the first BHJ active layer may share an electron donormaterial with the second BHJ active layer (e.g., the first electrondonor material may be the same as the second electron donor material).In some embodiments, the first BHJ active layer may have a distinctelectron acceptor material from the second BHJ active layer (e.g., thefirst electron acceptor material may be different than the secondelectron acceptor material). In some embodiments, the first BHJ activelayer may share an electron acceptor material with the second BHJ activelayer (e.g., the first electron acceptor material may be the same as thesecond electron acceptor material).

In some embodiments, a p-phenylene layer is formed prior to orsubsequent to block 3116. The p-phenylene layer may be formed by vapordepositing p-phenylene on a transparent electrode or on a differentbuffer layer. For example, the p-phenylene layer may be formed on a MoO₃layer in any of the embodiments described herein. In some embodiments,the material can be deposited from solution, electrochemically, orreactively grown on the surface. In some embodiments, other relatedmaterials can be used that achieve a similar efficiency enhancement. Therelated materials may have similar molecular properties of the disclosedp-phenylene (such as other oligo-phenylenes, or substitutedp-phenylenes) and they may be used in combination with other anodelayers or buffers instead of the ITO and/or MoO₃ layers describedherein. In some embodiments, the p-phenylene layer is used in an tandemarchitecture, in which the p-phenylene is deposited on top of theinterconnecting recombination layers, on the anode, or both, with thesame general effects for each subcell. In some embodiments, thep-phenylene layer is used in an inverted architecture, in which theanode is deposited on top of the photoactive layer with the same generaleffects. Other types of solar cells that may benefit from thep-phenylene layer may include lead-halide and other perovskites, quantumdot, and dye sensitized cells.

At block 3118, a compound charge transport layer is formed. The compoundcharge transport layer may be a compound hole transport layer or acompound electron transport layer. In some embodiments, the compoundcharge transport layer formed at block 3118 may be a second compoundcharge transport layer and the compound charge transport layer formed atblock 3110 may be a first compound charge transport layer. In someembodiments, forming the compound electron transport layer may includeforming a metal-oxide interlayer (block 3120) and forming a chargetransport layer (block 3122). The metal-oxide interlayer may be coupledto the photoactive layers and the charge transport layer may be coupledto the metal-oxide interlayer. The compound charge transport layerformed at block 3118 may comprise the same materials described inreference to the compound charge transport layer formed at block 3110.

At block 3124, an charge-injection layer is formed. The charge-injectionlayer may be coupled to the charge transport layer formed at block 3122.The charge-injection layer may be a hole-injection layer or anelectron-injection layer. The charge-injection layer may be ametal-oxide hole-injection layer or a metal-oxide electron-injectionlayer. In some embodiments, the charge-injection layer formed at block3124 may be a second charge-injection layer and the charge-injectionlayer formed at block 3108 may be a first charge-injection layer.

At block 3126, a second (e.g., top) electrode is formed, such as, e.g.,a second transparent electrode. The second transparent electrode may beformed on the charge-injection layer formed at block 3124 or on thephotoactive layers. The second transparent electrode may be formed usingtechniques applicable to formation of first transparent electrode atblock 3106.

At block 3128, one or more additional optical layers are optionallyformed, such as on the second transparent electrode.

Method 3100 may optionally be extended to correspond to a method forgenerating electrical energy. For example, a method for generatingelectrical energy may comprise providing a visibly transparentphotovoltaic device, such as by making a visibly transparentphotovoltaic device according to method 3100. Methods for generatingelectrical energy may further comprise exposing the visibly transparentphotovoltaic device to visible, ultraviolet and/or near-infrared lightto drive the formation and separation of electron-hole pairs, forexample, for generation of electrical energy. The visibly transparentphotovoltaic device may include the visibly transparent photoactivecompounds described herein as photoactive materials, buffer materials,and/or optical layers.

As used herein, “comprising” is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps. As usedherein, “consisting of” excludes any element, step, or ingredient notspecified in the claim element. As used herein, “consisting essentiallyof” does not exclude materials or steps that do not materially affectthe basic and novel characteristics of the claim. Any recitation hereinof the term “comprising”, particularly in a description of components ofa composition or in a description of elements of a device, is understoodto encompass those compositions and methods consisting essentially ofand consisting of the recited components or elements. The inventionillustratively described herein suitably may be practiced in the absenceof any element or elements, limitation or limitations which is notspecifically disclosed herein.

Abbreviations that may be utilized in the present specification include:

TPBi: 2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)HAT-CN:Dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrileTAPC: 4,4′-Cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine]C₆₀: Fullerene-C₆₀C₇₀: Fullerene-C₇₀ClAlPc: Chloroaluminum phthalocyanineF₈ZnPc: Zinc(II) 2,3,9,10,16,17,23,24-octafluorophthalocyanineF₁₆CuPc: Copper(II)1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadecafluoro-29H,31H-phthalocyanineZnO: Zinc oxidep-5P: Para-quinquephenylp-6P: Para-sexiphenyl

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention in the useof such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed. Thus, it should be understood that although thepresent invention has been specifically disclosed by preferredembodiments and optional features, modification and variation of theconcepts herein disclosed may be resorted to by those skilled in theart, and that such modifications and variations are considered withinthe scope of this invention as defined by the appended claims.

What is claimed is:
 1. An organic photovoltaic device comprising: asubstrate; a first electrode coupled to the substrate; a secondelectrode disposed above the first electrode; one or more photoactivelayers disposed between the first electrode and the second electrode;and a compound charge transport layer disposed between the one or morephotoactive layers and either the first electrode or the secondelectrode, wherein the compound charge transport layer includes: acharge transport layer; and a metal-oxide interlayer coupled to thecharge transport layer and disposed between the charge transport layerand the one or more photoactive layers.
 2. The organic photovoltaicdevice of claim 1, further comprising: a second compound chargetransport layer coupled to the compound charge transport layer, whereinthe second compound charge transport layer includes: a second chargetransport layer; and a second metal-oxide interlayer coupled to thesecond charge transport layer and disposed between the second chargetransport layer and the one or more photoactive layers.
 3. The organicphotovoltaic device of claim 1, wherein the first electrode is an anodeand the second electrode is a cathode.
 4. The organic photovoltaicdevice of claim 1, wherein the first electrode is a cathode and thesecond electrode is an anode.
 5. The organic photovoltaic device ofclaim 1, wherein the charge transport layer is a hole transport layer.6. The organic photovoltaic device of claim 1, further comprising: ametal-oxide charge-injection layer disposed between the first electrodeand the compound charge transport layer.
 7. The organic photovoltaicdevice of claim 1, wherein the charge transport layer is an electrontransport layer.
 8. The organic photovoltaic device of claim 1, furthercomprising: a metal-oxide charge-injection layer disposed between thecompound charge transport layer and the second electrode.
 9. The organicphotovoltaic device of claim 1, wherein the one or more photoactivelayers includes a bulk heterojunction photoactive layer comprising ablend of an electron donor material and an electron acceptor material.10. The organic photovoltaic device of claim 1, wherein the organicphotovoltaic device is visibly transparent.
 11. The organic photovoltaicdevice of claim 1, wherein the charge transport layer has distinctspectral absorption properties from the one or more photoactive layers.12. A method of making an organic photovoltaic device, the methodcomprising: providing a substrate; forming a first electrode over thesubstrate; forming a second electrode over the first electrode; formingone or more photoactive layers between the first electrode and thesecond electrode; and forming a compound charge transport layer betweenthe one or more photoactive layers and either the first electrode or thesecond electrode, wherein the compound charge transport layer includes:a charge transport layer; and a metal-oxide interlayer coupled to thecharge transport layer and disposed between the charge transport layerand the one or more photoactive layers.
 13. The method of claim 12,further comprising: forming a second compound charge transport layercoupled to the compound charge transport layer, wherein the secondcompound charge transport layer includes: a second charge transportlayer; and a second metal-oxide interlayer coupled to the second chargetransport layer and disposed between the second charge transport layerand the one or more photoactive layers.
 14. The method of claim 12,wherein the charge transport layer is a hole transport layer.
 15. Themethod of claim 12, further comprising: forming a metal-oxidecharge-injection layer between the first electrode and the compoundcharge transport layer.
 16. The method of claim 12, wherein the chargetransport layer is an electron transport layer.
 17. The method of claim12, further comprising: forming a metal-oxide charge-injection layerbetween the compound charge transport layer and the second electrode.18. The method of claim 12, wherein the one or more photoactive layersincludes a bulk heterojunction photoactive layer comprising a blend ofan electron donor material and an electron acceptor material.
 19. Themethod of claim 12, wherein the organic photovoltaic device is visiblytransparent.
 20. The method of claim 12, wherein the charge transportlayer has distinct spectral absorption properties from the one or morephotoactive layers.